import javax.swing.tree.TreeNode;

/**
 * Created with IntelliJ IDEA.
 * Description:
 * User:DELL
 * Time:23:32
 */
public class demo {
    long prev=Long.MIN_VALUE;
    public boolean isValidBST(TreeNode root) {
        if(root==null){
            return true;
        }
        boolean left=isValidBST(root.left);
        //剪枝
        if(left==false){
            return false;
        }
        boolean cur=false;
        if(root.val>prev){
            cur=true;
        }
        //剪枝
        if(cur==false){
            return false;
        }
        prev=root.val;
        boolean right = isValidBST(root.right);
        return left&&cur&&right;
}
